1. Field of the Invention
The invention relates to an improved device for generating in the ground both transverse and longitudinal acoustic waves in a plurality of different directions, useful in particular in the field of seismic prospection.
In seismic prospection, determining the position of the reflecting layers of the subsoil is often effected by transmitting into the ground longitudinal acoustic waves (P) which propagate in a compression mode. But transverse waves (S) are also used which propagate in a distortion or shearing mode. The transverse waves permit interesting applications. Since their wave length is, for equal frequencies, less than that of the longitudinal waves they have a better separating power. Furthermore, as is well known by specialists, the comparison of recordings obtained by transverse waves having different polarization directions makes it possible to detect fractures oriented in the subsoil and so new possibilities of interpretation.
2. Description of the Prior Art
A known device, adapted for generating transverse acoustic waves in the ground, is described in, for example French Pat. No. 2,276,599. It comprises essentially a target element having a surface for application on the ground and at least one lateral striking surface perpendicular to the application surface, a mass for striking the target element, an arm pivoting with respect to a platform for supporting the mass and operating means for causing the arm to pivot with respect to the platform between a low position in which the mass comes into contact with the lateral striking surface and a top position.
Lifting means fixed to a vehicle make it possible to move the target element between a transport position and a low position in which the application surface is in contact with the ground. With the target element disposed on the ground, the mass, previously raised by pivoting the arm, is released and strikes the lateral surface of the target element in a direction substantially parallel to the surface of the ground. Due to the shock, transverse acoustic waves are transmitted into the ground.
In French Pat. No. 2,530,344 an improvement to the preceding device is described in which the target element, the platform supporting the shaft of the pivoting arm and the operating means are connected to a rigid cage associated with guide means fixed to the chassis of the vehicle and movable in translation with respect thereto between a first working position in which the target element is in contact with the ground and a second raised position. Drive means make it possible to move the rigid cage with respect to the chassis between the first and the second positions and to press the target element against the ground in the working position.
The rigid cage has for example columns adapted for sliding vertically with respect to the chassis of the vehicle, with these columns being interlocked together by a first rigid structure supporting the platform and the operating means and a second rigid structure connected to the target element by deformable coupling means.
With the target element, the arm associated with the mass and the means for operating the arm connected to a rigid cage movable between a position in which the target element is pressed against the ground, this anchorage force being for example at least a part of the weight of the vehicle, and a raised position making it possible to move the vehicle, with the time interval separating two successive tripping operations at two different positions, being thus considerably reduced.
Furthermore, the striking faces are inner faces of the target element and the pivoting arm, the platform and the operating means are adapted so that the movements of the mass are circumscribed in a volume defined by the cage and the chassis of the vehicle. This arrangement makes it possible to limit the lateral dimension of the working zone and to prevent the access of staff inside the zone of movement of the mass during operation of the device.
Such a device is adapted for creating pure transverse waves and does not lend itself well to applications in which P and S wave seismic recordings are made.
From the U.S. Pat. Nos. 4,291,780 and 4,390,077, a device is known for applying pulses to the ground in an oblique direction so as to generate both longitudinal and transverse waves.
These pulses are generated by the impact of a mass falling with a rectilinear movement towards an impact face inclined with respect to the vertical of a target element coupled to the ground or towards another diametrically opposite face. The mass is guided in its fall by the parallel uprights of a guide mast orientable with respect to the target element. The impact speed of the mass is maximum when the mast is vertical, that is to say for the production of compression waves and decreases when it is slanted. Since the friction forces of the mass against the guide mast increase with the slant, it becomes more difficult to forecast with duration of the fall and so the precise moment of impact. Now, for numerous applications, the slant angle of the guide mast necessary so as to obtain a suitable amplitude ratio between the S and P waves, must reach high values. This operating condition is unfavorable to good synchronization.